1. Field of the Invention
The invention relates generally to acoustic sensors. More particularly, the invention relates to acoustic sensors that feature optical fiber wound around a compliant structure.
2. Description of the Related Art
Conventional fiber optic acoustic sensors frequently feature a sensing arm and a reference arm that terminate at an optical coupler. The sensing arm consists of a first optical fiber wound tightly around a compliant mandrel. The reference arm consists of a second optical fiber of fixed length disposed in an environment where stresses are minimal. Under quiescent conditions (no acoustic wave) light introduced into both the sensing arm and the reference arm travels through the respective fibers and arrives at the coupler. The path length of the sensing arm and the reference arm are fixed, thus light from each arm will arrive at the coupler with a time invariant phase difference. Under these conditions, the output of the coupler (mixed light) will be a light wave of constant amplitude.
If an acoustic wave is introduced into the environment of the sensing arm, the compliant mandrel will respond to the acoustic wave by expanding and contracting, stressing the sensing fiber. The stress on the sensing fiber changes the path length of light traveling through the fiber, modulating the light, accordingly. At the same time, the reference arm's path length remains unchanged in response to the acoustic wave. When light from both arms is mixed the light amplitude will varies proportionally with the incident acoustic wave.
One of the problems with conventional fiber optic acoustic sensors is that the sensitivity and directivity of the acoustic measurement is dependent on the size and orientation of the compliant mandrel. As the acoustic wavelength approaches the length of the mandrel, the sensor's sensitivity rapidly decreases. To maintain a constant frequency response over the entire band of frequencies of interest, hydrophone designers generally limit the length (and the diameter) of mandrels to half the wavelength of the highest frequency of interest.
This size limitation on the length of the mandrels imposes a practical limit on the operating band of fiber optic acoustic sensors. As the frequency of interest increases, the wavelength of interest decreases, requiring the use of very small mandrels. With very small mandrels, the number of fiber optic windings that can be formed around the mandrel decreases resulting in less sensitivity. In practice, this problem makes fiber optic acoustic sensors based on fiber wound mandrels a poor choice for sensing frequencies above 50 KHz.
Another problem with conventional fiber optic acoustic sensors is the frequency response is dependent on the direction of arrival of the acoustic wave. Compliant mandrels often are more responsive to acoustic waves that impact the mandrel broadside (radial direction of the mandrel) and less responsive to acoustic waves that impact the endfire (longitudinal direction of the mandrel). This is because the mandrel diameter is usually smaller than its length.
There currently is a need for fiber optic acoustic sensors that can detect frequencies up to 100 KHz with a constant sensitivity over the entire frequency band. There is also a need for a fiber optic acoustic sensor that is able to detect acoustic waves arriving at the sensor from any spatial direction.